Fretting fatigue of TI-6AL-4V: A micromechanical approach

Extending previous 2-D constitutive models, 3-D crystal plasticity models are developed for dual-phase Ti-6Al-4V and used to investigate and quantity the influence of microstructural heterogeneity on cyclic microplastic deformation and cumulative ratchet strain for representative fretting contact conditions. Fretting experiments, which involve reciprocating sliding of a cylindrical body over a flat body at small relative slip amplitudes, indicate that crack formation in the surface layers is affected by a plastic ratcheting mechanism. In an aerospace gas turbine engine, the dovetail joint between the blade and disk is prone to fretting. It is useful to examine the fretting fatigue problem using microstructurally-based models given that the length scales at which fretting fatigue cracks initiate are on the order of microstructural dimensions and initiation sites are likely to be influenced by the highly heterogeneous nature of these materials at this scale. Fretting simulations using a simplified 2-D planar triple slip crystal plasticity model, compared to simulations conducted using a conventional J₂ initially homogeneous plasticity model with non-linear kinematic hardening, indicate that crystal plasticity is necessary to capture the shear localization and dominant plastic ratchetting deformation mechanism in the near surface layers and hence provides much more realism in problems dealing with plastic deformation at fretting and sliding contacts. [1,2,3]. Idealized fretting contacts are analyzed to construct fretting maps that may be useful in design. The constitutive model is extended to 3-D and applied to behavior of an equiaxed duplex microstructure of Ti-6Al-4V consisting of primary α-phase grains and secondary α+β lamellar grains. The primary α-phase is of hep crystal structure and the α+β lamellar grains consist of alternating plate-like layers of a secondary a-phase and a bcc structured β-phase, which are subject to a burgers orientation relation. We explore implications of slip system anisotropy, incompatibility at grain and phase boundaries, microtexture, and phase distribution in relation to the plastic strain evolution during the fretting process.